1. Two methods are given for measuring the rate of diffusion of CO 2 in tissue membranes. Methods are also given for the determination of tissue thickness and the absorption coefficient for CO 2 in tissues. 2. The values obtained for the permeability constant ( P x 10 4 ) at 22°C. for CO 2 in the following tissues are:—frog skin, 3.05; connective tissue (dog), 2.65; smooth muscle (cat), 5.00; frog muscle, 5.29; striated muscle (dog), 4.70. P is expressed as cc. per cm. 2 per minute under a pressure gradient of one atmosphere per cm. 3. Evidence is presented to show that in a "steady state" bicarbonate contributes a negligible amount to the diffusion of CO 2 . 4. The absorption coefficient for CO 2 in frog skin is 0.73 cc. per cc. and for frog muscle 0.78 cc. per cc. 5. In all of the tissues studied the diffusion of CO 2 is slower than in water. The diffusion coefficients ( K x 10 4 in cm. 2 /minute) at 22°C. for tissues as compared with water are:—water (16°C.), 9.5 (Hüfner, 1897); frog skin, 4.1; connective tissue, 3.7; frog muscle, 6.8; striated muscle (dog), 6.0; smooth muscle (cat), 6.4. 6. The time course of saturation of a tissue with CO 2 is altered in the presence of available base. Non-acidified tissues saturate more slowly than acidified tissues and the rate of saturation is dependent on the CO 2 tension.